This invention relates to a small current detector circuit and a locator device that uses it. The small current detector circuit is specifically of a type that detects small currents (charge currents) which are generated upon charging or discharging of capacitive or electrostatic sensors such as capacitive position sensors, piezoelectric sensors, capacitive humidity sensors, static field sensors, electrostatic digitizers and locator devices. The locator device is of a type that detects the charge current difference in a pair of capacitors. When electrode pairs in a grid pattern are scanned as capacitor pairs with a finger, touch detection signals each having two peaks, one being greater and the other smaller than a specified reference level, are generated in the area around the touched electrodes and sampled from the small current detector circuit. The invention relates particularly to a locator device having electrodes arranged in a grid pattern on small pitch and which is capable of detecting touched positions with high accuracy compared to the width of finger touch and which requires no readjustments for each product in assembly line.
The locator device is used as a substitute pointing device for the mouse, track ball and quick pointer on a computer system. The locator device has an electrostatic sensor portion comprising multiple X and Y electrodes arranged in a grid pattern and the position of a touched electrode is detected by detecting the difference in capacitance between electrode pairs. For detecting the position of the touched electrode, X or Y electrodes are scanned with adjacent electrodes taken as a set. The difference in capacitance between two capacitors formed by a pair of electrodes and another electrode in a face-to-face relationship with the pair is detected as a difference in charge current by means of the small current detector circuit and output as a detection signal.
A prior art small current detector circuit for use with capacitive or electrostatic sensors is shown in FIG. 8 and may be described as an electrostatic sensor circuit, or a charge current detector circuit for detecting the charge current generated upon charging of a capacitor.
Referring to FIG. 8, the charge current detector circuit which is generally indicated by 9 has an electrostatic position sensor portion 1 having two capacitors Ca and Cb, one of which (Ca in the case under consideration) serves as a charge current detecting sensor, or a so-called "touch" sensor, having a surface that can be touched from the outside. The detector circuit 9 also includes a pulse drive circuit 2 which applies drive pulses to either terminal of each capacitor at specified periods. The other of the terminals of capacitors Ca and Cb are connected to the inverting inputs of operational amplifiers (OP) 3 and 4, respectively.
The operational amplifiers 3 and 4 are each an inverting amplifier, with its non-inverting input being connected to the ground. The output voltages VA and VB of the respective operational amplifiers are fed back to the inverting inputs via feedback capacitors C3 and C4. Initializing switches 5 and 6 are provided parallel to respective capacitors C3 and C4. Prior to a detecting operation, these switch circuits are turned on for a specified period in response to a control signal as from a controller or the like.
The output voltage VB of operational amplifier 4 is supplied via a resistor R to the inverting input of a buffer amplifier 7 which is capable of inverted amplification. The amplifier 7 has a feedback resistor R which is equal in value to the resistor R provided between the output of operational amplifier 4 and the inverting input of said amplifier 7, whereby the amplification factor of the buffer amplifier 7 is adjusted to unity. Hence, the output voltage VB of operational amplifier 4 is simply inverted to produce a negative voltage signal -VB, which is delivered as an output voltage from the amplifier 7.
The output voltage (-VB) of buffer amplifier 7 and the output voltage (VA) of operational amplifier 3 are summed in an adder 8 which is capable of inverted amplification. Since the buffer amplifier 7 produces an output which is an inversion of the output from the operational amplifier 4, what actually occurs is the substraction of the output (VB) of operational amplifier 4 from the output voltage VA of operational amplifier 3, with -(VA-VB) being produced as an output from the adder 8. In the circuit configuration described above, when a difference occurs between the capacitances of capacitors Ca and Cb, the quantity of the charge building up in one capacitor becomes different from that in the other capacitor, creating a difference in the flowing charging current. In response to this difference, a detection signal will accordingly be obtained at the output of the adder 8.
The detecting operation is the same irrespective of whether.it is accomplished by the operational amplifier 3 or 4 and, hence, the following description concerns only the operational amplifier 3. First, the switch circuit 5 is turned on for a specified period in the initial state. Since the inverting and non-inverting inputs of the operational amplifier 3 are virtually shorted, the turning on of the switch circuit 5 causes the output of the operational amplifier 3 to drop to the ground level (GND). As a result, the capacitor C3 is cleared by being discharged. At the same time, the capacitor Ca is similarly cleared by being discharged via the pulse drive circuit 2.
When the switch circuit 5 is turned off, a pulse signal is synchronously sent from the pulse drive circuit 2 to both capacitors Ca and Cb. The pulse signal passing through the capacitor Ca is applied to the inverting input of the operational amplifier 3, whereupon a current for charging the capacitor Ca flows in the path and thereby charges the capacitor Ca. The current flowing to the inverting input of the operational amplifier 3 is in proportion to the resulting charge buildup in the capacitor Ca. At the same time, a voltage output capable of holding the inverting input of the operational amplifier 3 at the ground potential develops at its output. In response to this output voltage, an electric current flows through the capacitor C3 to charge it. Since this charging operation occurs in such a direction that the operational amplifier 3 produces a negative output, the polarity of the capacitor C3 is as shown in FIG. 8, with the terminal to the inverting input of the operational amplifier 3 being positive and the terminal to the output being negative. As a consequence, the operational amplifier 3 produces the output voltage VA. Similarly, the operational amplifier 4 produces the output voltage VB.
Assume here that capacitor Ca is disposed in a specified detecting position and that its capacitance is changed by being touched by the operator or as the result of a metal coming close to it. If the capacitance of capacitor Cb does not change since it is a reference capacitor, the adder 8 provides an output spinal of a voltage level in inverse proportion to the change in the capacitance of the capacitor Ca. Thus, one can detect the touching of the capacitor Ca or the change in the position of the target.
Electrostatic digitizers, locator devices and other sensors typically employ the above-described charge current detector circuit as a basic circuit and comprise a matrix array of capacitors Ca each having an electrode that can be touched by the operator. The individual detecting capacitors Ca are sequentially selected by scanning with a multiplexer. The change in the capacitance of a selected capacitor that occurs relative to the adjacent capacitor as a result of touching or other events is detected in the manner just described above. Thus, the position of locating by touching or other events can be detected on the basis of multiplexer selection timing and the change in the capacitance of the selected capacitor.
In a charge current detector circuit having a detecting capacitor and a capacitor of a reference capacitance (or an adjacent capacitor), the detecting capacitor (Ca in FIG. 8) is usually provided in the detecting position, so it is wired to the operational amplifier 3 over a long distance. As a result, the capacitance of the capacitor Ca which is less than a hundred pF is highly sensitive to noise and the detected voltage will fluctuate to increase the chance of erroneous detection. In addition, the dynamic range of the detectable voltage is small since the change in an electric current to be detected is no greater than what develops in response to the change in capacitance due to an environmental change such as occurs when the operator touches the capacitor or if a metal object comes close to it. If the change in current is to be picked up by the operational amplifier, the offset of its operation will be a problem to the detecting operation.
To solve that problem, the Applicants previously invented a small current detector circuit which was less affected by noise in operation and they filed a U.S. patent application titled "Small Current Detector Circuit and Locator Device Using the Same", which was eventually granted as U.S. Pat. No. 5,783,951.
Returning to a locator device, X and Y electrodes in the electrostatic sensor portion are usually stripe electrodes thinner than the width of a finger which touches them. When an electrode is touched with a finger, the electric lines of force between X and Y electrodes are interrupted to reduce the capacitance of the touched electrode. As a result, there occurs a change in the difference of capacitance between the touched electrode and the adjacent one. When the difference in capacitance between an electrode pair and another electrode in a face-to-face relationship with the pair is successively detected by electrode-pair scanning with the small current detector circuit, the difference is positive and increases in the area upstream of the touched electrode. The difference then decreases and becomes zero in the position touched with a finger (the center of the finger) and thereafter increases taking a negative value. The difference then decreases to become zero again. This is the characteristic of the touch detection signal detected with the small current detector circuit. Briefly, the touch detection signal obtained by scanning X or Y electrode pairs with the small detector circuit varies in the scan direction in such a way that two peaks occur with reference to a specified level, one being greater and the other smaller (see FIG. 5a).
Since the touch detection signal obtained by electrode scan has such a waveform, the position of the touched electrode can theoretically be located by detecting the zero-crossing point between adjacent peaks. In practice, however, the electrostatic sensor portion comprises an array of X and Y electrodes of small width placed in a face-to-face relationship and the small variations that inevitably occur in the voltage applied to electrodes tend to deteriorate the S/N ratio. Particularly in the case where the capacitance between electrodes in the electrostatic sensor portion is very small on the pico order ranging from several to ten-odd picofarads, the waveform of the touch detection signal obtained by electrode scan is prone to be distorted and the reference level for the touch detection signal is subject to variations since the charge current to be finally detected is highly sensitive to the ambient. This lowers the precision of detection of the zero-crossing point between peaks. In addition, if the electrode pitch is very small (&lt;1,000 .mu.m) compared to the width of finger touch, it becomes even more difficult to locate the correct position of the touched electrode.
Under these circumstances, the locator device of the type of contemplated by the invention requires adjustments of the reference level and various circuits. In addition, variations are prone to occur between product lots and level adjustments are necessary for each lot. These facts all contribute to the increase in the number of production steps.